The master of suspense did not care whodunnit. For Hitch, the question was all but academic: to be alive is to stained. Culpability comes with conception.

It's hard to think of any adult in his films – the great ones, anyway – whose copybook has not a smudge, whose odd blots don't mushroom and bleed, soak their coats and cloud their judgement. And, for me, his primary preoccupation was never more brilliantly realised than in Strangers on a Train, the murder-swap thriller from 1951.

To refresh: straight-batting, social-climbing tennis star Guy (Farley Granger) has a chance encounter on a train with sardonic playboy Bruno (Robert Walker). Bruno knows Guy is romancing a senator's daughter (played by Ruth Roman) – and indeed is eager to move into politics – but can't get a divorce from his unfaithful wife, Miriam. Bruno himself is fed up with his controlling father. So he proposes they exchange bump-offs, to insulate the other from blame.

Guy laughs the idea off, but Bruno goes through with the killing, strangling Miriam in an amusement park. The appalled – yet liberated – Guy must then fend off the suspicions of the police, the fears of his fiancee and, most dangerous of all, Bruno, eager that Guy goes through with his end of the bargain. And, as the original trailer hammers home – even as it sternly asserts this is a fable of how evil can infect the clear conscience – it is a bargain.

Strangers on a Train

Right from the start, we see that Guy does, on some level, accede. It's an absolutely audacious sequence, so thematically cohesive, so snappy, so fun. In that, it absolutely bears the thumbprints of Hitch, rather than, say, Patricia Highsmith, on whose novel it's based, or even Raymond Chandler, whose credit was flagged energetically in publicity, but whose early draft was junked by the director, who roped in his wife and Ben Hecht's assistant, Czenzi Ormonde, to overhaul.

That opener is a true coup de cinema; there are countless others threaded through. The trip through the underworld as Bruno tops Miriam after hopping in a boat called Pluto through the tunnel of love. It's never fantastical – filmed in an actual fairground – but it's absolutely operatic in its nods and shots, its sillouettes and soundtrack (Miriam and her two groping friends actually sing along to the song 'The Band Played On'). The climatic fight on the carousel; the murder refracted in the victim's spectacles; the Expressionist visuals – Hitch's most vivid use of swoops and chiaroscuro since the drummer scene in Young and Innocent).

A word for the grace notes, too: Bruno's manicure from his mother; his hound at the top of the stairs, his nail-biting grope for the lighter dropped down a drain, spliced with Guy's epic, against-the-clock tennis match, which must be completed before Bruno plants it as incriminating evidence.

All these suggest our sympathies should lie with Bruno, not our hero. So does the casting. Robert Walker is everything Kevin Spacey could be; deft and feline, smart and charming, sweet and insidious. That he was just 31 when he shot this is shocking; that he died eight months later, suffering from mental illness, is terrible.

Strangers on a Train, directed by Alfred Hitchcock. Photograph: taken from picture library

Conversely, Granger does well to make our clean-cut leading man so much of a drag. How different would it have been had James Stewart taken that part, or Cary Grant. Granger is dapper, like Stewart, but nervous where Stewart was lackadaisical. Unlike Grant, he is permitted, in this, no discernible sense of humour, and only surface, mannequin glamour. Ruth Roman, taking third billing, was foisted on an unconvinced Hitch, and it shows. The movie seems to mock her while appearing to idolise. The lighter may be the MacGuffin but the girl is an irrelevance, a token, an afterthought.

For the key to Strangers is that it's a love story; Bruno and Guy's meeting in the dining car is every bit as seductive as Grant and Marie Saint's locomotive bunkup in North by Northwest. This is a film saturated in the sweat of McCarthyism (there's an astonishing shot of Bruno, alone, clad in black, watching Guy from the steps of the Jefferson Memorial).

Strangers on a Train

Even the final scene, a supposedly larky little coda warning against the perils of chat with even the most innocuous fellow passenger, would not work were we still not jittery with a paranoia that's part and parcel of just pottering around in the world. Whoever you are, whatever your situation, whispers can be insistent, skeletons have a habit of rattling the handle.

• This article was amended on 3 August 2012 to correct the spelling of Marie Saint and Pluto.